taru-adjective, adverb taking the 'to' particle
Archaic literary word describing darkness or gloom, often used in classical or poetic contexts. Modern equivalents include 暗い or 薄暗い.
古い文章で「暗暗とした森」という表現を見かける。
In old texts, you come across the expression 'a dark, gloomy forest'.
夜の暗暗たる道を一人で歩く。
Walking alone on a dark, gloomy road at night.
Reduplication of 暗 (an, 'dark') with the reading あん, forming an archaic emphatic or adverbial form. The exact historical derivation is uncertain.